Process-monitoring devices are used to detect defects during the laser-machining of workpieces. For example, when laser-welding workpieces, fusion defects may occur in the weld seam in which a flush connection is not made in certain areas between the workpieces welded to one another. Since, in the event of such fusion defects, the weld seam appears to be free of defects when viewed from the outside, indirect assessment parameters usually have to be used to detect these weld seam defects clearly.
WO 2008/145237 A1 discloses, in this respect, a method in which a detector oriented coaxially to the laser beam is used to detect, in a two-dimensionally spatially resolved manner, radiation emitted by the workpiece in the detection field of the sensor. The weld seam defects are detected by evaluating the previously detected radiation in the area of a solidified melt adjoining a liquid weld pool and/or in the area of the liquid weld pool of the weld seam. In this method, a linear weld seam is welded so that the focal spot of the laser beam and the weld seam generated have a fixed, known (central) position in the detection field of the camera on the workpiece, i.e. its image capture area on the workpiece. The linear course of the weld seam allows a simple evaluation of the camera image because only limited image areas at previously fixed, always identical points in the image have to be evaluated.
In practice, however, cuts or weld seams having freely programmable, e.g., curved shapes, are also generated in laser-machining processes. When cutting or welding any desired shape of path, the position of the cutting gap, the weld pool and the seam in the camera image cannot be predicted for image evaluation because it depends on the feed direction of the machining head. For this reason, in order to detect defects, the entire detection field of the photo detector must be evaluated in each case in order to be sure of detecting the position of the gap or the weld seam. Depending on the optical and temporal resolution (image refresh rate) of the detectors, this can be an extremely complex, computationally intensive and time-consuming process. This problem may be exacerbated further in the case of a combined movement of the machining head and scanner optics arranged therein with respect to the workpieces to be machined (which is not uncommon in practice, for example, in the case of remote laser welding).
DE 10 2008 062 866 A1 discloses a method for detecting defects in a weld seam produced using the above-mentioned laser scanner welding process. After the weld seam has been produced, heat radiation emitted by the workpiece is detected using a camera oriented coaxially to the laser beam as described above and defects in the weld seam are detected based on an evaluation of the detected heat radiation. With this method, it is not possible to intervene in a closed-loop control of the welding process if a defect is found in the weld seam. The laser-welding process also has to be interrupted whenever the weld seam is checked, which can be a disadvantage in terms of time and cost.
DE 10 2007 025 463 A1 discloses a laser scanner welding method in which a camera oriented coaxially to the laser beam is likewise arranged in the detection beam path of the scan head in order to monitor the welding process. The laser-machining of parts of workpieces is carried out in quick succession at several machining positions of the workpieces. To allow essentially real-time monitoring of the machining process at the individual machining points, the acquisition of images by the camera is in each case triggered by the control signal of the scanner. The individual images taken are then assigned to different machining sequences, respectively, so that an image sequence is produced for each machining point. These sequences can subsequently be evaluated separately.
DE 10 2007 024 510 B3 discloses a method for the real-time monitoring of a laser scanner machining process, in which a desired fracture line is made in a workpiece using a laser beam. On one side of the workpiece facing away from the laser beam, the respective machining area, i.e. the desired fracture points to be made in the workpiece along a predefined machining path, is imaged by a matrix camera that is located at a fixed position. To monitor the laser-machining process, only pixels of the CMOS detector of the matrix camera for which the impingement of measurement radiation is expected are read and evaluated. This means that the computational effort and time required to evaluate the detected radiation can be reduced. This method is relatively inflexible because the detector is spatially fixed and not coaxial to the laser beam. For example, the stationary measurement sensor cannot be used in robotics-based remote laser welding processes, such as when making passenger car bodies.